Flavonoids are polyphenolic compounds that occur ubiquitously in foods of plant origin and are well known for their antioxidant capacities. Major dietary sources of flavonoids are vegetables, fruits, and beverages such as tea and red wine. Among the dietary flavonoids, quercetin-glycosides are amongst the most abundant. Flavonoids in general have been reported to confer a number of health benefits and are believed to act by intervention in various metabolic pathways such as by inhibition of 5-cyclooxygenase. Included within the general term flavonoid are flavonols, flavones, flavanones, catechins, anthocyanins, isoflavonoids, dihydroflavonols and stilbenes.
The main types of flavonols found in plants are based on quercetin, kaempferol and myrecetin, and their respective glycosides.

This figure depicts the three different flavonol aglycones (no sugars attached). The sugars are usually attached to the 3 and 7 positions, but attachments on the 4′ and 3′ and possibly even 5 positions feature as well. The sugars are either attached as monomers, dimers and sometimes trimers. Sugars include glucose, rhamnose, galactose, xylose and arabinose. More than one attachment site can be used, although 4 sugars appears to be the maximum number observed. Flavonols represent a large class of molecules all based on a small number of core structures and natural variation is achieved by attachment of other molecular entities e.g. sugar, methyl groups etc, at different positions of the flavonol core-ring structure. Glycosylated forms are very abundantly found in nature, although the un-glycosylated form (aglycon) can occur as well.
Different plants have different profiles of flavonol glycosides. For example, onions are rich in quercetin-3,4′-diglucoside and quercetin-4′-glucoside. In addition, they contain smaller amounts of 3-glucoside, 4′,7-diglucoside and of rutin (3-rutinoside). Apples contain rutin, quercetin-3-galactoside, quercetin-3-arabinofuranoside, quercetin-3-glucoside, quercetin-3-rhamnoside, quercetin-3-xyloside, quercetin-3-arabinoside. Tea contains rutin as the main flavonol, but also contains quercetin-3-glucoside, quercetin-3-galactoside, quercetin-3-rhamnoside-diglucoside. Buckwheat contains high levels of rutin in the leaves and flowers and is the main commercial source for rutin supplements on the market. Tomato contains rutin as the main flavonol. Broccoli and kale are good sources of quercetin-glycosides and contain even more kaempferol-glycosides (about twice the amount of quercetin-glycosides). Kaempferol-glycosides are routinely found in many plants alongside quercetin-glycosides but often, although not always, in much smaller quantities.
Onions, mainly yellow and red onions, are the food crops with the highest natural levels of quercetin-glycosides and typically contain about 300-600 mg/kg fresh weight (FW) of flavonols. Similar, albeit slightly lower levels are present in berries such as cranberries, lingonberries, bilberries and blackcurrants. Other major sources are apples which can have up to 100 mg/kg FW and tea which can have about 25 mg per cup of tea. Tomatoes when unmodified typically contain about 10 to 20 mg flavonols per kg FW, prototype high flavonol tomato varieties have been shown to contain 350 mg/kg FW, whilst concentrated tomato paste made from such prototypes contains about 1200 mg/kg FW.
In unmodified tomato fruits, the main flavonoid found is naringenin chalcone (Hunt et al, Phytochemistry, 19, (1980), 1415-1419). It is known to accumulate almost exclusively in the peel and is simultaneously formed with colouring of the fruit. In addition to naringenin chalcone, glycosides of quercetin and, to a lesser extent, kaempferol are also found in tomato peel.
Verhoeyen M. et al “Increasing antioxidant levels in tomatoes through modification of the flavonoid biosynthetic pathway” J Exp Botany (2002) 377: 2099-2106, outlines the various approaches to enhance flavonoid biosynthesis in tomatoes. Methods for increasing the production of flavonoids in plants by manipulating gene activity in the flavonoid biosynthetic pathway are disclosed in WO-A-99/37794, WO-A-00/04175 and EP 1254960.
An elevated blood pressure or hypertension has a prevalence of about 15% in Western populations and is increasing in developing countries. Above the age of 65 the incidence increases to approximately 35%. Hypertension is an established and independent risk factor for coronary heart disease (CHD), kidney and heart failure and stroke and may lead to disability and premature death. Lowering blood pressure in hypertensive subjects is effective in reducing the risk and disability of associated diseases. Specifically, published epidemiological studies have shown that lowering blood pressure in humans by even a few mmHg reduces the incidence of several cardiovascular diseases. For example, lowering systolic blood pressure by 5 mmHg reduces all-cause mortality by 7% on a population basis, while coronary heart disease and stroke was reduced by 9 and 14%, respectively (Whelton et al. (2002) JAMA 288:1882-1888).
Spontaneously Hypertensive Rats (SHR) are considered to be a representative model of human essential hypertension. These rats are generally used to understand the development and establishment of hypertension and to determine the blood pressure lowering effect of newly synthesised anti-hypertensive drugs. In a recent study by Duarte et al., “Effects of chronic quercetin treatment on hepatic oxidative status of spontaneously hypertensive rats” Mol. Cell Biochem (2001) 221:155-160, it was shown that SHR are characterised by increased hepatic and plasma malondialdehyde concentrations, indicating increased oxidative stress. Duarte's group further found that treatment of SHR with quercetin aglycone reduced blood pressure, increased glutathione peroxidase activity and reduced both plasma and hepatic malondialdehyde levels. It was concluded that quercetin aglycone therefore shows both antihypertensive and antioxidant properties in this model of genetic hypertension (SHR).